Homogeneous Electron-Transfer Reactions Studied by Internal Reflection Spectroelectrochemistry

Using the technique of internal reflection spectroelectrochemistry with subsequent signal averaging, two types of fast homogeneous electron-transfer reactions have been studied. In the reproportionation type where the reacting pairs differ by two electrons (A₁ + A₁^2± ⇌ 2A₁±) an electrochemical potential perturbation was applied to transform through heterogeneous transfer, A₁ to A₁^2±, at a diffusion-controlled rate at the electrode surface. The A₁^± species was usually optically monitored during this perturbation to relate concentration in the reaction layer to the kinetics. Values of K_eq for the reactions were conveniently evaluated from the differences in the peak potentials obtained by cyclic voltammetry. In the other cross-electron-transfer reaction (A₁ + A_2± ⇌ A₁^± + A₂), two molecular pairs (1 and 2) and their one-electron ions are involved. Particular attention was given to a series of ferrocene molecules reacting with a single redox couple, tri-p-anisylamine and its radical cation in the solvent of acetonitrile. The general magnitudes for the rate constants were correctly predicted by the Marcus theory and, for the ferrocene series, the correct relative rates were found. The rates were predominantly dependent on the standard free energy for the reacting redox pairs. Rates varying over a range of six orders of magnitude are reported. Thin platinum films deposited on glass or quartz were used as optically transparent electrodes in this work.